PBRT wouldn't finish building on FreeBSD

[michaelcw02]

Hi,

I'm currently trying to use PBRT v3 on several operating systems and one of them is FreeBSD. While trying to install PBRT I found several issues. The issues that I found are:

1. Issue on PTEX project because alloca.h was not found. I have raised an issue and PR on that repo.
2. After the above has been circumvented, there was an issue related to the following test: Linking CXX executable stacktrace_unittest. I was only able to solve it by removing the following lines of the Google Logging Library. In this commit are the changes that I did.